The present invention relates to an optical transmission system comprising a transmitter for transmitting optical pulses via an optical transmission medium to a receiver, the receiver comprises optical clock recovery means for generating a sequence of optical pulses with a frequency related to a pulse frequency of the optical pulses received from the transmission medium, the optical clock recovery means comprises an optical amplifier having its input coupled to its output via a feedback loop, said feedback loop comprises a non linear element, the optical clock recovery means comprises injection means for injecting the optical signal received from the optical transmission medium.
The invention further relates to an optical receiver, clock recovery means and a non-linear optical element.
Such a transmission system is known from the article xe2x80x9c10 Gbit/s all-optical regeneratorxe2x80x9d by W. A. Pender et. al. in Electronics Letters 31st August 1995, Vol. 31, No. 18. pp. 1587-1588.
The transmission rate of optical transmission systems is increasing rapidly, due to improvements of optical components such as lasers, modulators, multiplexers and demultiplexers. The transmission rate approaches the limit of capabilities of the used electrical components such as photo detectors and modulators. To overcome this limitations there is a trend to use time division multiplex in which a plurality of optical signals with a bitrate of e.g. 10 Gbit/s are multiplexed into an optical signal with a higher bitrate e.g. 40 Gbit/s. This multiplexing can completely be performed by optical means.
In the receiver the received optical signal has to be demultiplexed by optical means because using electrical components for handling such a high bitrate signal is very difficult. Such optical demultiplexing requires the availability of an optical clock signal. Because this clock signal has to be synchronized to the received input signal, the receiver comprises clock recovery means to extract the required optical clock signal from the received signal. In the optical transmission system according to the above mentioned article, clock recovery means using only optical components are used.
The clock recovery system according to the above mentioned article is based on mode locking of a ring-laser in which the received optical signal is injected. A ring laser comprises an optical amplifying device having its output coupled to its input by means of a feedback loop. Mode locking is a phenomenon that results in the generation of optical pulses having a repetition rate which is a multiple of the reciprocal of the round trip delay of the combination of amplifier and feedback loop. The phenomenon is caused by non-linear amplitude and phase transfer in the feedback loop. To obtain said non-linear amplitude and phase transfer a non-linear element is present in the feedback loop.
In the transmission system according to the above mentioned article the non-linear element comprises 1 km dispersion shifted fiber followed by a polarizer. A first non-linear effect comprises a change of the polarization state in dependence of the amplitude of the optical signal in the 1 km dispersion shifted fiber. A second non-linear effect is a phase shift in the fiber which is dependent on the amplitude in the fiber. The polarizer is adjusted such that it passes the light signals only for a polarization state corresponding to the polarization state for a signal with a high amplitude.
A problem with the clock recovery system according to the above mentioned article is that it requires 1 km of optical fiber, making integration of said system impossible.
The object of the present invention is to provide a transmission system according to the preamble of which the receiver has substantially smaller dimensions than the receiver according to the prior art.
To achieve said object, the present invention is characterized in that said non linear element comprises a four port coupling element, two ports of the coupling element being included in the feedback loop and two ports being included in a secondary loop, the coupling element being arranged for coupling a first optical signal into a first branch of the secondary loop and for coupling a second optical signal into a second branch of the secondary loop, the secondary loop being arranged for causing signal level dependent interference between the first and second optical signal when re-entering the coupling element
By using a secondary loop in which two optical signals propagate in opposite directions, it becomes possible to use level dependent interference between said two optical signals to obtain a non-linear operation. If e.g. for a first level of the optical signals non-destructive interference occurs and for a second level destructive interference occurs it is obtained that the non-linear element passes signals having the first level, and attenuates signals having the second level. A non-linear element according to the invention can have much smaller dimensions than the non-linear element used in the prior art transmission system.
It is observed that the article xe2x80x9cAll-optical clock recovery using a modelocked figure eight laser with a semiconductor non-linearityxe2x80x9d by L. Adams, E. Kintzer and J. Fujimoto in Electronics Letters No. 20, Vol. 30, Sep. 29, 1994 an optical clock recovery system is disclosed in which a primary and a secondary loop is used. In this system however, the input signal of the receiver is injected into the secondary loop instead of into the feedback path of the amplifier. This has as consequence that the laser will not be modelocked when the input signal is absent, but the laser will generate a CW signal. This is in general undesirable in a time domain multiplexed system, because it causes the input signal to be passes to all tributary outputs. It has further been shown that the pulse rate of the xe2x80x9cFIG. 8xe2x80x9d laser is lower and the pulse width is higher than the corresponding values of the system according to the invention.
An embodiment of the invention is characterized in that the first branch of the secondary loop is coupled to a first port of a further optical amplifier, in that the second branch of the secondary loop is coupled to a second port of the optical amplifier and in that the delay values of the branches of the secondary loop are different.
An optical pulse entering the coupling element will split in two optical pulses propagating in the secondary loop in opposite directions. The optical pulse propagating through the branch with the shorter delay value will arrive earlier at the amplifier.
If the level of the optical pulse exceeds a given level, the amplifier will be saturated and the gain will decrease. When the second pulse arrives at the amplifier, this pulse will be amplified by a smaller factor that the first pulse. Consequently the two optical pulses will have different amplitudes when re-entering the coupling element. Also the phases will be different due to the amplitude dependent phase shift in the amplifier. If this phase difference is properly chosen non-destructive interference will occur in the coupling element.
If the level of the optical pulses is small, the amplifier is not influenced by the optical pulses. Consequently both pulses remain equal after having propagated in the secondary loop. If the properties of the coupling element are chosen correctly, destructive interference will occur for such low level pulses.
A further embodiment of the invention is characterized in that the amplitude transfer factors of the branches of the secondary loop are different.
By making the amplitude transfer functions of both parts of the secondary loop different, it is prevented that the amplifier is also saturated by the second pulse, causing the amplifier faster to recover. This results in a faster non-linear element.
A still further embodiment of the invention is characterized in that the secondary loop comprises an attenuator to make the amplitude transfer factor of the first branch of the secondary loop different from the amplitude transfer factor of the second branch of the secondary loop.
By using an attenuator for obtaining different amplitude transfer factors for the parts of the secondary loop it is obtained that the dimensions of the secondary loop can be substantially reduced because the different attenuation factors do not have to be obtained by different fiber lengths.